various process IDs, including those of background jobs
(see Lists), the value of $$, and the value of
$PPID

When a simple command other than a builtin or shell function
is to be executed, it
is invoked in a separate execution environment that consists of
the following. Unless otherwise noted, the values are inherited
from the shell.

the shell’s open files, plus any modifications and additions specified
by redirections to the command

the current working directory

the file creation mode mask

shell variables and functions marked for export, along with variables
exported for the command, passed in the environment (see Environment)

traps caught by the shell are reset to the values inherited from the
shell’s parent, and traps ignored by the shell are ignored

A command invoked in this separate environment cannot affect the
shell’s execution environment.

Command substitution, commands grouped with parentheses,
and asynchronous commands are invoked in a
subshell environment that is a duplicate of the shell environment,
except that traps caught by the shell are reset to the values
that the shell inherited from its parent at invocation. Builtin
commands that are invoked as part of a pipeline are also executed
in a subshell environment. Changes made to the subshell environment
cannot affect the shell’s execution environment.

Subshells spawned to execute command substitutions inherit the value of
the -e option from the parent shell. When not in POSIX mode,
Bash clears the -e option in such subshells.

If a command is followed by a ‘&’ and job control is not active, the
default standard input for the command is the empty file /dev/null.
Otherwise, the invoked command inherits the file descriptors of the calling
shell as modified by redirections.